1. Field of the Invention
The invention relates to a capacitor of high thermal stability for carrying out measurements on a gas-insulated high-voltage line, including a high-voltage electrode extending in a longitudinal direction, a cylindrical printed circuit coaxially surrounding said high-voltage electrode and including an electrically conductive track forming a low-voltage electrode, and a frame having a cylindrical inside surface coaxially surrounding said cylindrical printed circuit, which is held by said frame.
2. Background Art
A capacitor of this kind is intended in particular for metal-clad electrical switchgear, i.e. switchgear insulated by a gas such as SF6, for example. It is used to measure the voltage of a high-voltage member such as a cable of a shielded high-voltage line. The high-voltage member is electrically connected to the high-voltage electrode of the capacitor and the frame of the capacitor is electrically connected to ground. A system of this kind is installed in an electrical distribution substation at the output of electrical power generation plant, for example.
The high-voltage electrode of the capacitor, which generally consists of a round-section metal rod, is surrounded by the low-voltage electrode, which is fixed to the frame but electrically insulated from it. The frame of the capacitor is connected to ground to form a capacitive divider bridge between the high-voltage electrode and ground. To be more specific, the divider bridge includes a first capacitor consisting of the high-voltage electrode and the low-voltage electrode and which is connected in series with a second capacitor consisting of the low-voltage electrode and the frame. The voltage at the low-voltage electrode is therefore an image of the high voltage and the low-voltage electrode is connected to a measuring device.
With an arrangement of the above kind, the measured voltage values are in particular a function of the capacitance C of the first capacitor, which is given by the well known equation:
                    C        =                  K          ·                      S                                          D                ·                I                            ⁢                                                          ⁢                              n                ⁡                                  (                                      D                    d                                    )                                                                                        (        1        )            in which K is a constant, D and S are the diameter and the surface area of the low-voltage electrode, and d is the diameter of the high-voltage electrode. This type of capacitor is generally installed outdoors and is subject to temperature variations from −40° C. to +80° C. These fluctuations cause expansion or contraction of the materials from which it is made, which causes its capacitance to vary in wide proportions, since the dimensions of the materials change in accordance with the well known equation:L(T)=L0(1+a.(T−T0))  (2)in which L(T) is a length at temperature T, L0 the same length at a reference temperature T0 and a is the coefficient of linear expansion of the corresponding material. An increase in temperature increases the surface area S and the diameter D of the low-voltage electrode, which varies the capacitance of the capacitor, because of equations (1) and (2). Thus temperature fluctuations induce capacitance variations that interfere with the accuracy of the measurements.
In designing this kind of capacitor, the aim is therefore to minimize the effect of temperature on the capacitance to obtain thermal stability yielding satisfactory measurement accuracy. Optimization generally consists in designing a particular mechanical architecture that is associated with a choice of materials having suitable coefficients of expansion.
French patent FR-2705492 discloses a capacitor of the kind referred to above in which the low-voltage electrode consists of a plurality of curved plates along the inside periphery of the frame. The plates are fixed to the frame but free to expand or contract. In this prior art capacitor, the variation in the diameter of the low-voltage electrode is therefore conditioned by the coefficient of expansion of the material from which the frame is made, because the plates are fixed to the frame. Variations in the surface area of the low-voltage electrode are conditioned by the coefficient expansion of the material or materials of the plates. By combining equations (1) and (2) it can be shown that thermal stability is satisfactory if the ratio of the coefficient of thermal expansion of the material of the frame to that of the material of the low-voltage electrode is from 1.5 to 2.5. The above prior art capacitor has satisfactory thermal stability but is costly to manufacture because of the necessity to fix a plurality of plates to the inside periphery of the frame.
German patent DE-2409595 discloses a cylindrical capacitor enclosing a cylindrical printed circuit whose inside face is coated with a metal layer to form the low-voltage electrode. This form of construction with a cylindrical printed circuit for the low-voltage electrode is very simple and reduces the cost of fabrication. However, the cylindrical printed circuit is free to expand and contract inside the frame when the temperature varies, and this makes the capacitor too sensitive to temperature variations.
The aim of the invention is to remedy these drawbacks by proposing a capacitor of high thermal stability and simple construction.